Ryan C. Nieuwendaal

Supramolecular Self-Assembly of a Model Hydrogelator: Characterization of Fiber Formation and Morphology

Hydrogels are of intense recent interest in connection with biomedical applications ranging from 3-D cell cultures and stem cell differentiation to regenerative medicine, controlled drug delivery, and tissue engineering. This prototypical form of soft matter has many emerging material science applications outside the medical field. The physical processes underlying this type of solidification are incompletely understood, and this limits design efforts aimed at optimizing these materials for applications. We address this general problem by applying multiple techniques (e.g., NMR, dynamic light scattering, small angle neutron scattering, rheological measurements) to the case of a peptide derivative hydrogelator (molecule 1, NapFFKYp) over a broad range of concentration and temperature to characterize both the formation of individual nanofibers and the fiber network. We believe that a better understanding of the hierarchical self-assembly process and control over the final morphology of this kind of material should have broad significance for biological and medicinal applications utilizing hydrogels.

Formation of extended ionomeric network by bulk polymerization of l,d-lactide with layered-double-hydroxide

Abstract We report the formation of an ionomeric network in a poly( l , d -lactide) hybrid nanocomposite, (PLDLA-HYB) during in-situ melt polymerization of l , d -lactide in the presence of magnesium/aluminum layered-double-hydroxide (LDH) without added catalyst. The effect of LDH mass loading and reaction time on molecular mass and yield of soluble poly( l , d -lactide) (PLDLA-SOL) present in the hybrid was investigated for a better understanding of the conflicting roles of LDH in polymerization and degradation of PLDLA-SOL. High molecular mass PLDLA-SOL is obtained through initiation of polymerization by LDH. However an additional insoluble organic–inorganic fraction, INSOL, is also observed within the product when PLDLA-SOL is extracted using methylene chloride as solvent. It is proposed that INSOL is an ionomeric network comprising hydrogen-bonded, or otherwise co-ordinated, lactic acid monomer salts of magnesium, together with PLDLA in a 24%–76% mass ratio.

On the role of experimental imperfections in constructing 1H spin diffusion NMR plots for domain size measurements

We discuss the precision of 1D chemical-shift-based (1)H spin diffusion NMR experiments as well as straightforward experimental protocols for reducing errors. The (1)H spin diffusion NMR experiments described herein are useful for samples that contain components with significant spectral overlap in the (1)H NMR spectrum and also for samples of small mass (<1mg). We show that even in samples that display little spectral contrast, domain sizes can be determined to a relatively high degree of certainty if common experimental variability is accounted for and known. In particular, one should (1) measure flip angles to high precision (≈±1° flip angle), (2) establish a metric for phase transients to ensure their repeatability, (3) establish a reliable spectral deconvolution procedure to ascertain the deconvolved spectra of the neat components in the composite or blend spin diffusion spectrum, and (4) when possible, perform 1D chemical-shift-based (1)H spin diffusion experiments with zero total integral to partially correct for errors and uncertainties if these requirements cannot fully be implemented. We show that minimizing the degree of phase transients is not a requirement for reliable domain size measurement, but their repeatability is essential, as is knowing their contribution to the spectral offset (i.e. the J1 coefficient). When performing experiments with zero total integral in the spin diffusion NMR spectrum with carefully measured flip angles and known phase transient effects, the largest contribution to error arises from an uncertainty in the component lineshapes which can be as high as 7%. This uncertainty can be reduced considerably if the component lineshapes deconvolved from the composite or blend spin diffusion spectra adequately match previously acquired pure component spectra.

Effect of temperature on the structure and dynamics of triblock polyelectrolyte gels

Triblock polyelectrolyte gels were characterized by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The oppositely charged end blocks self-assemble into polyelectrolyte complex cores, while the neutral poly(ethylene oxide) middle block bridges adjacent cores. The size of the polyelectrolyte complex core does not change with temperature. However, the neutral middle block displays a temperature-dependent conformation. The liquid-like order of the complex core within the gel phase leads to stretched bridging chains that approach their unperturbed dimensions with increasing concentration. A stretch ratio for bridging chains was defined as the ratio between stretched and unperturbed dimensions. A further reduction in the chain stretching occurs with increasing temperature due to solvent quality. DLS observes multiple modes consistent with a collective diffusion (fast mode) and diffusion of clusters (slow mode). The dynamics of these clusters are at length scales associated with the SANS excess scattering, but with relaxation time near the crossover frequency observed by mechanical spectroscopy.

Chapter 7:Structure and Order in Organic Semiconductors

Several techniques are presented for characterizing the order of semiconducting polymers on different length scales, including: differential scanning calorimetry; solid-state nuclear magnetic resonance spectroscopy; transmission electron microscopy; and grazing incidence scattering. As the behavior of highly conjugated polymers can differ distinctly from the more classic, highly saturated polymers for which some of these techniques were originally developed (e.g. polyethylene), the hazards and pitfalls associated with applying these techniques to semiconducting polymers are also discussed.